ECE 170

Laboratory #5 Digital Oscilloscope

Lab #5 Digital Oscilloscope This lab is an introduction to the oscilloscope you will use in the rest of the core lab courses. This scope operates in a distinctly different manner from the analog oscilloscope. It is a hybrid of a digital voltmeter and a computer. A digital voltmeter is used to measure the voltage present at the input at a given point in time. This value is then passed to the computer, which stores it. After a time interval, another voltage is measured (sampled) and is stored. The computer uses these data points to generate a graph, which is displayed on the CRT. If the time interval is short in comparison to the change in the voltage, the graph will look like a solid line, similar to the analog display. However, it is important to remember that it is a set of discrete points. This distinction may not seem important now (nor is it very obvious), but in later classes the distinction will become more important. Since the voltage measurements are stored in the memory of the computer, the computer can measure different quantities for you. It can give voltage, frequency, and duty cycle measurements at the touch of a button. It can also store and recall displays. It can average several displays. It can even print out the display. All of these "bonus" functions may suggest that the analog scope is a dinosaur soon to die. However, since the two scopes operate slightly differently, the analog scope will be here for some time. We chose to introduce the analog scope first, because it required you to understand the operation of the scope and set the voltage and time scales as well as triggering manually. The digital scope can automatically set up the scope for you and display a trace it thinks you want. Most of the time, it is the signal you want, but occasionally you will need to set the controls manually. This is where this scope can be quite confusing.

Basic Concepts Before you begin, carefully look over Figure 5.1, and compare it with the front panel of the oscilloscope. These knobs and buttons are the "keyboard" that allows you to communicate with the computer. The knobs act in the same fashion as the knobs on the analog scope. The gray buttons act as main menu selectors, bringing up different menus for the softkeys (under the screen). Remembering which gray button you need to press to get the appropriate softkey can be very confusing. Figure 5.2 and Figure 5.3 show you how to obtain all the softkey functions. This lab will introduce you to many of the different softkeys, if you have a few minutes after lab, see what the others do. Another piece of equipment you will use this week is a Probe. Commonly, 10:1 probes are made and used. These probes reduce the voltage presented to the scope by a factor of ten. Thus, when using a 10:1 probe on most analog scopes, you must remember to multiply the voltage measurement by a factor of ten. (A 20Vpp sine wave will appear on the scope as a 2Vpp signal.) The digital scope (and some fancy analog scopes) will perform the multiplication for you. It is a good practice to measure a known voltage source to see if the multiplication needs to be done or not. Probes are used to match the characteristics of the scope with the device being measured. They also increase the

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ECE 170

Laboratory #5 Digital Oscilloscope

apparent internal resistance of the scope by a factor of 10. 100:1 probes exist and are used for very high voltage work. 1:1 probes also exist, but are unusual in the form of the probes that we have here. In fact, you have already used a 1:1 probe. What was it? Probes are expensive ($75–$100), and are easily damaged with improper use. The probe has a grounding cord dangling off of it. When hooking up the ground strap to ground, make sure it is ground. Numerous probes have met an early retirement (been destroyed), because the user connected the ground strap to a power supply rather than ground. The probe usually has a spring-loaded cover on it, which allows the probe to be clipped to a post or wire. This is how we will use it. The cover can come off exposing a sharp point, which can be used to touch different pins on an IC (or prod a sleeping lab partner). In a pinch, it is tempting to place this point in the hole of a BNC--DON'T DO IT. The point breaks off very easily and can not be replaced. We will also use a training board designed to highlight all the different features of the HP 54600 series oscilloscopes. This device has several different function generators on it, all of which can be accessed by touching or clamping onto the posts along the outside edge. Note that the ground posts are labeled on the board in gold. This board is made using surface mount technology, which allows for the components to be placed quite closely together and doesn't require holes to be drilled through the board. Do you see any resistors on the board? Ask your instructor to point out a few--they're the small brown rectangles with silver ends. (So much for color bands...) The black packages with eight or ten leads sticking out of them are called integrated circuits or ICs. Don't try touching the leads of the ICs with the probe you have—it isn't fine enough, and if you hit two pins at the same time you can destroy the IC!

STORAGE Measure Volta ge

Tim e

Run

Cur sors

S ave/Recall Tra ce

Autostor e

Stop

HORI ZONTAL

Erase

TRIGGER

Setup Sour ce

Aut oscale

Disp lay

__

__

Mo de

Main De layed

V ERTICAL Vo lts/Div

Level

Del ay

Pr int Utility

Volts/Div

T im e/Div

TV

__

Ho ldoff

Slope

Co u pli ng

5V

2 mV

5 V

1

2

Position

Positio n

1X 5 V 0V - 1 .2 k Hz

Line 0

2 mV

2Y !

1 Må - 13 pF 4 00 V Ma x

1

Figure 5.1: HP 54600 Digital Oscilloscope.

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Extern al Tr igger

Z ! 1 Må - 13 pF 400 V M ax

ECE 170

Laboratory #5 Digital Oscilloscope

Key Pressed 1 or

Menu Obtained 1 Off On

Coupling DC AC

BW Lim Off On

Invert Off On

Vernier Off On

Probe 1 10 100

Roll

Vernier Off On

Time Ref Lft Cntr

2

Main Delayed

Main

Horizontal Mode XY Delayed

Trigger Source Source

1

2

Line

Trigger Mode Auto Lvl

Auto

Normal

DC AC

Off

Single

TV

Mode

Slope Slope

Reject LF

HF

Noise Rej Off On

Coupling

Print Utility

Cursor

Print Screen

Source 1 2

Clear Menu

V1

Self Tst Menu

Active Cursor V2 t1

Figure 5.2: Function Keys Part 1.

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Self Cal Menu

t2

Clear Cursors

ECE 170

Laboratory #5 Digital Oscilloscope

Menu Obtained

Key Pressed

Display

# Average Display Mode Normal Peak Det Average 16 64 256

Display Mode Setup

Trace

Voltage

Next Menu

Time

Next Menu

1

Save

V p-p

Show Meas Off On

Vmax

Show Meas Off On

Clear Mem1

Voltage Measurements

Source 1 2

Source 1 2

Undo Autoscale

Recall

Trace Trace Mem1 Save to Mem 1 Mem2 Off On Mem1

V avg

V rms

Default Setup

Recall Setup

Clear Meas

Voltage Measurements Vmin Vtop Vbase

Time Measurements Freq Period Duty Cy

+Width

Grid Off On

Clear Meas

Next Menu

Previous Menu

Next Menu

Time Measurements Previous -Width RiseTime FallTime Menu

Figure 5.3: Function Keys Part 2.

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ECE 170

Laboratory #5 Digital Oscilloscope

Equipment HP 54600 Oscilloscope Two 10:1 probes. HP 54654a self-paced training board for the HP 54600 series oscilloscope. HP Training guide

Instructional Objectives 5.1

5.2 5.3 5.4

For a digital oscilloscope, demonstrate the use of power on, amplifier on, x-y operation, variable sensitivity control, position, ground/AC/DC selection, digital measurements, and trace storage. Explain the differences between a digital and an analog oscilloscope. Using the training board, display an ellipse on the digital oscilloscope. Measure the phase difference between two sinusoidal waveforms on the digital oscilloscope.

Procedure Work through the HP Training Guidebook to obtain each of the signals on the training board on the scope. As you work through the book, review the following questions and write down the answers as you go along. Make sure to display all the signals and steps covered in the book even if there are no questions asked about that section.

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ECE 170

Laboratory #5 Digital Oscilloscope

Name________________________

Section: ____________________

ECE 170 Lab 5 Digital Oscilloscope 1.

Work through pages 16 – 18 of the training manual. You will only need to compensate your probe if your screen does not look like Figure 5 in the training manual. How does changing the probe attenuation factor change the display? __________________________________________________________________ How does it change your voltage readings? __________________________________________________________________ While your probe is connected up to the test point, touch the ground strap to the test point. What happens to the display? __________________________________________________________________ If the display hasn’t changed, your ground strap is broken and needs to be replaced. You should always check your probes prior to use as ground straps often break. They are purposely made to be a weak point in order to act as a fuse and protect the probe.

2.

Work through pages 19 – 21 of the training manual. What does the vernier function allow you to do? __________________________________________________________________

3.

Work through pages 22 – 25 of the training manual. What does the delayed mode allow you to do? __________________________________________________________________ What buttons need to be pressed in order to change the slope of the trigger? __________________________________________________________________

4.

Work through pages 26 – 30 of the training manual. By how much does signal #1 lead signal #2 (express your answer in degrees)? __________________________________________________________________

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ECE 170

Laboratory #5 Digital Oscilloscope

What is the ∆V for part 6 on page 28 of the training manual? __________________________________________________________________ What is the average voltage of signal #1 using the automatic voltage measurement? The RMS Value? The peak-to-peak value? __________________________________________________________________ 5.

Work through pages 31 – 34 of the training manual. What is the period of signal #3 using the automatic measurements? __________________________________________________________________ What is the period of signal #3? __________________________________________________________________ What does the autostore function allow you to see for signal #4 and why? __________________________________________________________________

6.

Work through pages 35 – 37 of the training manual. autoscale allow you to do?

What does the undo

__________________________________________________________________ 7.

Work through pages 38 – 39 of the training manual. What is the value of the peak voltage of signal #6? __________________________________________________________________

8.

Work through page 40 of the training manual. What does the HF reject function do? __________________________________________________________________

9.

Work through page 41 of the training manual. What does the scope display for this signal when the autoscale button is pressed? __________________________________________________________________

10.

Work through pages 42 – 46 of the training manual. What do you have to know about the properties of a single shot waveform in order to view it on the scope? __________________________________________________________________

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ECE 170

Laboratory #5 Digital Oscilloscope

How did the display of signal #9 change when both signals 9 and 10 were displayed when compared to when signal #9 was displayed by itself? __________________________________________________________________ 11.

Work through pages 47 – 49 of the training manual. What does averaging do? __________________________________________________________________

12.

Work through pages 50 – 54 of the training manual. For two sine waves of equal magnitude, displayed in XY mode, what does a single diagonal line leaning to the right say about the phase difference between the two signals? __________________________________________________________________ For two sine waves displayed in XY mode, what does an oval with major and minor axis on the X and Y axis respectively, say about the oscilloscope settings? __________________________________________________________________

After you have finished using the training board, disconnect the 9V battery.

Lab Report Format for Lab #5 Due to the hands on nature of this experiment, you are not required to prepare a traditional lab report. All you will have to do is turn in pages 5-5 through 5-7 of this manual with your answers recorded in the space provided. Turn the pages in before you leave the lab.

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ECE 170

Laboratory #5 Digital Oscilloscope

Name: ____________________

Section: ____________________

Pre-Lab #6: Accuracy and Precision 1. The accuracy of a measuring instrument is given by the manufacturer as ± 1%. The digital readout shows a measured value of 36.212V. a) What is the range of uncertainty due to the stated accuracy?

b) What is the range of uncertainty implied by the precision of the reading?

c) What would be a better way to write the value based on the accuracy?

2. Two voltage measurements are taken in a circuit. V1 = 47V and V2 = 45V. Determine the voltage difference V1 – V2, indicating a range of uncertainty.

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